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LibJS: Make the forward transition chain weakly cached

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Before this patch, every shape would permanently remember every other
shape it had ever transitioned to. This could lead to pathological
accumulation of unused shape objects in some cases.

Fix this by using WeakPtr instead of a strongly visited Shape* in the
the forward transition chain map. This means that we will now miss out
on some shape sharing opportunities, but since this is not required
for correctness it doesn't matter.

Note that the backward transition chain is still strongly cached,
as it's necessary for the reification of property tables.

An interesting future optimization could be to allow property tables
to get garbage collected (by detaching them from the shape object)
and then reconstituted from the backwards transition chain (if needed.)
This commit is contained in:
Andreas Kling 2021-05-17 21:25:57 +02:00
parent 8c96640157
commit e0493c509e
2 changed files with 24 additions and 9 deletions
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22
Userland/Libraries/LibJS/Runtime/Shape.cpp
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@ -1,5 +1,5 @@
/*
* Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
* Copyright (c) 2020-2021, Andreas Kling <kling@serenityos.org>
*
* SPDX-License-Identifier: BSD-2-Clause
*/
@ -23,10 +23,23 @@ Shape* Shape::create_unique_clone() const
return new_shape;
}
Shape* Shape::get_or_prune_cached_forward_transition(TransitionKey const& key)
{
auto it = m_forward_transitions.find(key);
if (it == m_forward_transitions.end())
return nullptr;
if (!it->value) {
// The cached forward transition has gone stale (from garbage collection). Prune it.
m_forward_transitions.remove(it);
return nullptr;
}
return it->value;
}
Shape* Shape::create_put_transition(const StringOrSymbol& property_name, PropertyAttributes attributes)
{
TransitionKey key { property_name, attributes };
if (auto* existing_shape = m_forward_transitions.get(key).value_or(nullptr))
if (auto* existing_shape = get_or_prune_cached_forward_transition(key))
return existing_shape;
auto* new_shape = heap().allocate_without_global_object<Shape>(*this, property_name, attributes, TransitionType::Put);
m_forward_transitions.set(key, new_shape);
@ -36,7 +49,7 @@ Shape* Shape::create_put_transition(const StringOrSymbol& property_name, Propert
Shape* Shape::create_configure_transition(const StringOrSymbol& property_name, PropertyAttributes attributes)
{
TransitionKey key { property_name, attributes };
if (auto* existing_shape = m_forward_transitions.get(key).value_or(nullptr))
if (auto* existing_shape = get_or_prune_cached_forward_transition(key))
return existing_shape;
auto* new_shape = heap().allocate_without_global_object<Shape>(*this, property_name, attributes, TransitionType::Configure);
m_forward_transitions.set(key, new_shape);
@ -88,9 +101,6 @@ void Shape::visit_edges(Cell::Visitor& visitor)
visitor.visit(m_prototype);
visitor.visit(m_previous);
m_property_name.visit_edges(visitor);
for (auto& it : m_forward_transitions)
visitor.visit(it.value);
if (m_property_table) {
for (auto& it : *m_property_table)
it.key.visit_edges(visitor);